When a sample of crystalline silicon is covered with silicon
dioxide, the oxide-layer acts as a barrier to the diffusion of impurities, so that
impurities separated from the surface of the silicon by a layer of oxide do not
diffuse into the silicon during high-temperature processing. A p-n junction can
thus be formed in a selected location on the sample by first covering the
sample with a layer of oxide [oxidation step] removing the oxide in the
selected region, and then performing a predeposition and diffusion step. The
selective removal of the oxide in the desired area is performed with
photolithography. Thus, the areas over which diffusions are effective are
defined by the oxide layer with windows cut in it, through which diffusion can
take place. The windows are produced by the photolithographic process. This
process is the means by which microscopically small electronic circuits and
devices can be produced on silicon wafers resulting in as many as 10000
transistors on a 1 cm x 1 cm chip.
In fact photolithography or optical lithography is a kind of
lithography. The lithography technique was first used in the late 18th century by people interested in
art. A lithograph is a less expensive picture made from a flat, specially
prepared stone or metal plate and the lithography is art of making lithographs.
Therefore, lithography for IC manufacturing is analogous to the lithography of
the art world. In this process the exposing radiation, such as ultraviolet (UV)
light in case of photolithography, is transmitted through the clear parts of
the mask. The circuit pattern of opaque chromium blocks some of die radiation.
This type of chromium/glass mask is used with UV light. Other types of exposing
radiations are electrons, X-rays, or ions. Thus for IC manufacturing we have
following types of lithography. Photolithography has been explained in this
post. To know about the other types of lithographic process,.
- Photolithography
- Electron-beam Lithography
- X-ray lithography
- Ion-beam lithography
In IC fabrication a number of masks are employed. Except for the
first mask, every mask must be aligned to the pattern produced by the previous
mask. This is done using mask aligner. The mask aligner may be contact type or
proximity type or projection type. Accordingly we have three types of printing.
They are
§ Contact
printing
§ Proximity
printing
§ Projection
printing